Investigators say 27-year-old Martino Castro Jr. is the last of 21 people convicted and going to prison.
The case involves gun purchases for weapons being exported to Mexico and used by violent drug gangs. Five shipments were intercepted during the summer of 2010.
A federal judge in San Antonio on Tuesday sentenced Castro, who in December 2010 pleaded guilty to smuggling firearms. The San Antonio man also pleaded guilty to being a felon in possession of a firearm with an obliterated serial number.
Castro in 2009 was convicted in Del Rio of transportation of illegal immigrants and sentenced to nine months in prison.
